The present invention concerns a method for magnetic resonance imaging of a contiguous region of a subject, applying a partial parallel acquisition (PPA) via modulation of spin magnetization by way of radio-frequency pulses, as well as via spatial coding of the subject region and via subsequent measurement in cycles of the radio-frequency response signals showing the excited spins, based on the measurement of the radio-frequency response signals by a coil array comprising two or more radio-frequency receiver coils (via which both the coil sensitivity information and the radio-frequency response signals are acquired), whereby each radio-frequency receiver coil acquires a reduced data set for implementation of the image reconstruction.
The inventive method is also called by the acronym CAIPIRINHA (Controlled Folding In Parallel Imaging Results In Higher Acceleration) for reasons described below.
Magnetic resonance tomography (MRT) is based on the physical phenomenon of nuclear magnetic resonance and is used as an imaging method in medical diagnostics and in biophysics. As a “non-invasive” examination method with versatile contrast capabilities for slice-by-slice imaging of body organs, MRT has developed into a method that is in many aspects superior to x-ray computer tomography (CT). It is also important that in MRT, in comparison to CT, the patient undergoes no damaging radiation exposure.
In spite of the technical advances in the sequence methodology and in the hardware development, the acquisition time required for an MRT image remains a limiting factor. The development of methods to shorten the image measurement times is a goal because boundaries are placed on a further increase of the technical capacity (magnetic field strengths and pulse strengths) of MRT devices solely for reasons of patient protection (stimulation and tissue heating). A faster MRT imaging and “real-time imaging” to support minimally-invasive surgery have many advantages. With these technologies, expensive and high-risk examinations (such as heart catheter examinations) can be avoided.
The following explanation provides background for central terms and describes the experimental principle of MRT as well as the events necessary for image creation. In MRT, the examination subject is exposed to a strong, constant external magnetic field. In the body to be examined, the nuclear spins of the atoms that were previously oriented in various directions align according to the external magnetic field (spin magnetization). Radio-frequency waves of suitable frequencies, generated by additional coils, can excite the aligned nuclear spins to a specific oscillation. The signal to be measured is generated via this oscillation, which is registered with the aid of one or more receiver coils (see, e.g., Roemer P B, Edelstein W A, Hayes C E, Souza S P, Mueller O M. The NMR phased array. Magn. Reson Medical 1990; 16: 192–225).
With the aid of gradient coils, inhomogeneous magnetic fields (gradient fields) can be generated that coordinate in “pulse sequences” that enable a free selection of a slice to be imaged of the human body or, respectively, of an organ (spatial coding) to be imaged. For spatial coding, gradients are used that show in all three spatial directions. One differentiates the slice selection by the gradients Gz and the frequency coding (also designated as read coding) and the phase coding by respectively one of the two gradients Gx or Gy. The gradient Gz typically establishes an acquisition layer in the direction of the z-axis. The coding direction of the gradients Gx and Gy (which are orthogonal to one another) lie within the slice selected by Gz and allow (together with Gz) a complete spatial coding of the body region to be examined. For this, the measurement signals of the same y-coordinates are provided with the aid of a phase coding gradient with the same phase shift (phase displacement).
Upon readout, a third gradient is switched that ensures all regions with the same x-coordinates have the same frequency upon the data acquisition. The third gradient, here Gx, is also designated as a read gradient or frequency coding gradient.
In MRT, the measurement data is acquired in “k-space”, which is spanned by the spatial coding. The data of an individual k-space line are, as already noted, frequency-coded by way of a gradient upon readout. The separation of the lines in k-space Δky is normally generated via phase coding. K-space is associated with the image space via the mathematical operation of the Fourier transformation. The Fourier transformation of a function yields a decomposition of the function into periodic components of the wave number k. K-space is also called Fourier space or spatial frequency space.
The mathematical operation of folding additionally plays an important role in the framework of MRT image reconstruction. Folding or in-foldings are artifacts that, for example, always occur when the selected image region (FOV) is smaller than the extent/measure of the subject; or if, for example, a plurality of slices are simultaneously excited, one slice folds on the other and they can no longer be clearly separated. Altogether, “folding effects” are created due to such superimposition of various subject regions that should appear separated for imaging. These are also designated as “folding artifacts”, or folding, and occur in particular in the acquisition of reduced data sets under application of the partial parallel imaging.
This method (which simultaneously uses a plurality of coils for data acquisition) may be called partially parallel acquisition (PPA) and is explained more precisely later as the basis of the most frequent MRT methods. Given the selection of the phase coding direction via a gradient, for example Gy, folding also taken into account in the MRT experiment, meaning the folding is recognizable as such in the planning of the experiment. In connection with the inventive method, it has been recognized that an additional, monitored modulation of the magnetization allows the control of the folding.
In the past, the shortening of the measurement times was enabled via a number of improvements. These include the introduction of faster spin echo sequences and gradient echo sequences, or the development of the FLASH pulse sequence (Haase et al., 1986), which enabled a clear reduction of the time interval (TR) between the nuclear magnetic resonance excitations repeated during the MRT measurement. Naturally, the shortening of the measurement times was also enabled via the constant technical development of the hardware of MRT devices. The PPA imaging used in connection with the inventive method also belongs to this area.
The PPA imaging is characterized in that, via linear algebra, a plurality of incomplete MRT data sets of a subject that are, however, acquired simultaneously can be reconstructed into a complete image via the mathematical process of unfolding (see Bydder M, Larkman D J, Hajnal J V Generalized SMASH imaging. Magn Reson Med. 2002 January;47(1):160–70, Carlson J W, Minemura T. Imaging time reduction through multiple receiver coil data acquisition and image reconstruction. Magn Reson Med 1993; 29: 681–688, Carlson J W. An algorithm for NMR imaging reconstruction based on multiple RF receiver coils. J Magn Reson 1987; 74: 376–380, Griswold M A, Jakob P M, Heidemann R M, Nittka M, Jellus V, Wang J, Kiefer B, Haase A. Generalized autocalibrating partially parallel acqusitions (GRAPPA). Magn Reson Med 2002; 47: 1202–1210, Griswold M A, Jakob P M, Nittka M, Goldfarb J W, Haase A. Partiallay parallel imaging with localized sensitivities (PILS). Magn Reson Med 2000; 44: 602–609, Heidemann R M, Griswold M A, Haase A, Jakob P M. VD-AUTO-SMASH imaging. Magn Reson Med 2001; 45:.1066–1074, Hutchinson M, Raff U. Fast MRI data acquisition using multiple detectors. Magn Reson Med 1988; 6: 87–91., Jakob P M, Griswold M A, Edelman R R, Sodickson D K. AUTO-SMASH, a Self-Calibrating technique for SMASH imaging. MAGMA 1998; 7: 42–54, Kellman P, Epstein F H, McVeigh E R. Adaptive sensitivity encoding incorporating temporal filtering (TSENSE). Magn Reson Med. 2001 May;45(5):846–52., Kellman P, McVeigh E R. Ghost artifact cancellation using phased array processing. Magn Reson Med. 2001 August;46(2):335–43., Kelton J R, Magin R L, Wright S M. An algorithm for rapid image acquisition using multiple receiver coils. In: Proceedings of the 8th Annual Meeting of SMRM 1989: 1172., Kwiat D, Einav S, Navon G. A decoupled coil detector array for fast image acquisition in magnetic resonance imaging. Med Phys 1991; 18: 251–265., Kwiat D, Einav S. Preliminary experimental evaluation of an inverse source imaging procedure using a decoupled coil detector array in magnetic resonance imaging. Med Eng Phys 1995; 17: 257–263., Kyriakos W E, Panych L P, Kacher D F, Westin C F, Bao S M, Mulkern R V, Jolesz F A. Sensitivity profiles from an array of coils for encoding and reconstruction in parallel (SPACE RIP). Magn Reson Med. 2000 August;44(2):301–8., Larkman D J, Hajnal J V, Herlihy A H, Coutts G A, Young I R, Ehnholm G. Use of multicoil arrays for separation of signal from multiple slices simultaneously excited. J Magn Reson Imaging. 2001 February;13(2):313–7. McKenzie C A, Yeh E N, Ohliger M A, Price M D, Sodickson D K. Self-calibrating parallel imaging with automatic coil sensitivity extraction. Magn Reson Med. 2002 March;47(3):529–38., Pruessmann K P, Weiger M, Bornert P, Boesiger P. Advances in sensitivity encoding with arbitrary k-space trajectories. Magn Reson Med. 2001 October;46(4):638–51., Pruessmann K P, Weiger M, Scheidegger M B, Boesiger P. SENSE: sensitivity encoding for fast MRI. Magn Reson Med. 1999 November;42(5):952–62. , Ra J B, Rim C Y. Fast imaging using subencoding data sets from multiple detectors. Magn Reson Med 1993; 30: 142–145., Sodickson D K, Manning W J. Simultaneous acquisition of spatial harmonics (SMASH): Fast imaging with radiofrequency coil arrays. Magn Reson Med 1997; 38: 591–603, Sodickson D K, McKenzie C A. A generalized approach to parallel magnetic resonance imaging. Med Phys. 2001 August;28(8):1629–43., Wang J, Kluge T, Nittka M, Jellus V, Kuehn B, Kiefer B. Parallel Acquisition Techniques with Modified SENSE Reconstruction mSENSE. In: Proceedings of the First Wurzburg Workshop on Parallel Imaging. Pg. 92 (2001), Weiger M, Pruessmann K P, Boesiger P. 2D SENSE for faster 3D MRI. MAGMA. 2002 March;14(1):10–9). The coil geometry established by the assembly of the component coil comprising a plurality of individual coils thereby determines the folding effects occurring during the data acquisition.
The spatial separation of the measurement region and the simultaneous time-saving acquisition of partial images in PPA-MRT is experimentally enabled in that a plurality of individual coils that form a “coil array” are arranged around the subject to be examined. Each of the spatially independent coils of the array registers certain spatial information that is acquired as coil sensitivity information. This information is used in order to achieve a complete spatial coding via the combination of the data simultaneously acquired by a plurality of individual coils. A PPA acquisition typically results in folded images, whereby a folded image data set exists for each individual coil. The selection of the phase coding direction establishes the direction in which the folding effects occur in the subject space, and therewith in the image space. Special reconstructions that effectively correspond to an unfolding of the folded data set are then applied to the reduced data in order to reconstruct the missing image information.
In a PPA acquisition, conventional standard sequences (gradient echo, spin echo, EPI, True FISP, etc.) are utilized using radio-frequency pulses and magnetic field gradients; however, in comparison to the conventional acquisition, only a fraction (½, ⅓, ¼, etc.) of the phase coding lines are acquired. The complete image is thus obtained in a fraction of the time. The shortening of the image measurement time achieved via the PPA corresponds to the ratio of the number of lines of the complete data set to the number of lines of the reduced data set and is characterized by a reduction factor R.
However, given PPA imaging for a specific location in the examination subject, a complete reconstruction is only possible when clear sensitivity differences between the individual coils exist at this location. A robust reconstruction thus assumes an optimally large number of individual coils in two or three spatial directions, whereby the number of coils is also always limited by the state of technology. This problem restricts the PPA imaging in a disadvantageous manner with regard to the signal-to-noise ratio.
The further problems of the PPA imaging results from the fact that the PPA methods are exclusively oriented to the reconstruction of the data after completion of the acquisition. Given the SENSE (sensitivity encoding for fast MRI) method belonging to the prior art, the matrix inversion is exclusively applied after the data acquisition, in the framework of the postprocessing. Monitoring mechanisms that modify and control the folding space and folding effects during the actual data acquisition are unknown as of yet.